This invention relates to band reject filters, and in particular, to an electronically tunable band reject filter that operates in the Radio Frequency (rf) range.
A conventional tunable band reject filter, and in particular, a three-stage band reject or notch filter that is used in the prior art includes three series tuned shunt tank circuits that are separated by a transmission line that is approximately one-quarter wave length at the mid-band of frequency to which the filter is designed to operate. Very high tank Q's, the quality of merit of filters and tuned circuits, are required to obtain a high level of attentuation at the center frequency, particularly if linear phase shift response is required to within a few percent of the notch frequency.
An electronically tuned band reject filter may be constructed by using varactor diodes. Unfortunately, varactors do not have the extremely high Q of the air dielectric capacitors that they replaced. The reduced tank Q may not be acceptable, especially in applications where phase response is an important parameter.
The varactor tuned band reject filter is also limited to low power levels. At the notch, or center frequency, the resonant tanks present a very low impedance to the signal that is to be filtered. Since nearly all the incident power is reflected back to the signal source by the first pole, the voltage across the varactor in that pole is equal to the incident voltage times the Q of the tank. The peak swing across a varactor diode must be limited to several volts; consequently, the incident voltage must be on the order of ten millivolts or less.
One of the major disadvantages of the prior art band reject filter is that it reflects incidental power back to the load; hence, it has an infinite Voltage Standing Wave Ratio (VSWR) at the center frequency.